Biophysical controls on rhizospheric and heterotrophic components of soil respiration in a boreal black spruce stand

Abstract

We conducted a root-exclusion experiment in a 125-year-old boreal black spruce (Picea mariana (Mill.) BSP) stand in 2004 to quantify the physical and biological controls on temporal dynamics of the rhizospheric (Rr) and heterotrophic (Rh) components of soil respiration (Rs). Annual Rr, Rh and estimated moss respiration were 285, 269 and 57 g C m-2 year-1, respectively, which accounted for 47, 44 and 9% of Rs (611 g C m-2 year-1), respectively. A gradual transition from Rh-dominated (winter, spring and fall) to Rr-dominated (summer) respiration was observed during the year. Soil thawing in spring and the subsequent increase in soil water content (θ) induced a small and sustained increase in Rh but had no effect on Rr. During the remainder of the growing season, no effect of θ was observed on either component of Rs. Both components increased exponentially with soil temperature (Ts) during the growing season, but Rr showed greater temperature sensitivity than Rh(Q10) of 4.0 and 3.0, respectively). Temperature-normalized variations in Rr were highly correlated with eddy covariance estimates of gross ecosystem photosynthesis, and the correlation was greatest when Rr was lagged by 24 days. Within diurnal cycles, variations in Ts were highly coupled to variations in Rh but were significantly decoupled from Rr. The patterns observed at both time scales strongly suggest that the flow of photosynthates to the rhizosphere is a key driver of below-ground respiration processes but that photosynthate supply may control these processes in several ways. © 2008 Heron Publishing.